Dear Bob Just a point - smaller is not always better. AVX and maybe some others make 0612 capacitors for decoupling. They are the same size as the normal 1206 size but are metallised along their long edges.
This reduces their ESL considerably, to the extent that (according to manufacturer's data I have seen) their ESR performance can be better than the same value in an 0805 style, or even in an 0603. I haven't had the opportunity to try any 0612s out yet. Has anyone any real comparative data on these decaps? All the very best! Keith Armstrong Cherry Clough Consultants www.cherryclough.com In a message dated 18/04/02 00:11:45 GMT Daylight Time, bh...@ma.ultranet.com writes: > Subj:RE: Decoupling - capacitor values > Date:18/04/02 00:11:45 GMT Daylight Time > From:<A HREF="mailto:bh...@ma.ultranet.com";>bh...@ma.ultranet.com</A> > To:<A HREF="mailto:am...@westin-emission.no";>am...@westin-emission.no</A>, <A > HREF="mailto:emc-p...@majordomo.ieee.org";>emc-p...@majordomo.ieee.org</A> > Sent from the Internet > > In selecting that 820 pF capacitor, check the specifications. The > self-resonance frequency is a function of the capacitor value and the > physical size. Chip caps that are smaller physically will have a have a > self-resonant frequency higher than the same value cap with larger physical > dimensions. > > Regards, > > Bob Howland > dBH Consulting > > -----Original Message----- > From: owner-emc-p...@majordomo.ieee.org > [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of > am...@westin-emission.no > Sent: Wednesday, April 17, 2002 4:50 PM > To: emc-p...@majordomo.ieee.org > Subject: Decoupling - capacitor values > > > > A microprocessor is driven by a 12MHz clock. The 5V Vcc-pin is decoupled by > a 100nF capacitor with a few mm leads. We can observe an unwanted 156MHz > signal on the 5V line, maybe 13th harmonic of 12MHz. We will try to > suppress/decouple this 156MHz signal. > > Suggestion: > Insert a SMD ceramic capacitor of value 820pF in parallel with the existing > 100nF. The reason for the low value 820pF is because the capacitor > self-resonance frequency is approximate 180MHz, and I believe it is > important to choose a Cap value with a resonance frequency higher than the > frequency we would like to decouple. > > Does it make sense? > > Best regards > Amund Westin, Oslo/Norway
